Trays for holding papers and the like are common fixtures in commercial and home offices. A useful desk paper tray includes a surface upon which the tray contents can rest as well as front, back and side walls for restraining the contents within the tray. In certain trays, one or more wall is absent, or reduced in size, to facilitate access to the tray and its contents. A wide variety of such trays are available. Typically, such trays are designed so that a plurality of single level trays may be stacked one atop the other or may be otherwise joined together for use.
The vast majority of such trays are formed by an injection molding process. Typically, in commercial molded plastic products, the cycle time of the mold which makes the product is one of the most significant factor in the cost of the molded product produced. Virtually all such desk trays have been molded as single bin trays using one or more cavities in an injection mold. Conventionally in such molds the base or planar paper carrying portion of the tray is planar with the junction between the main two halves of the mold. That geometry, combined with the required physical size of the tray has made the power, or tonnage, of the injection molding machine into which the mold is place a significant limitation on the number of cavities which can be simultaneously molded. Within these limitations, the available strategy to reduce cost has been to reduce the wall thickness of the molded tray, which reduces the part weight and thus the cost of the raw material. This process has now reached the point at which it is optimized for this type of mold.
It is believed that, owing to the physical constraints inherent in an injection molding process, no injection moldable multilevel letter tray has heretofore been manufactured. It has therefore been desired to produce a low-cost multilevel tray in a single cycle using a single two part mold in a single or multi-cavity mold assembly. Two part molds do not have "side actions" or "cams" which are secondary mold parts that approach the mold cavity from the side and which increase the cost of the mold. Until now, most single bin desk trays have been manufactured from molds using such side actions, which allows the tray to be made with a stackable feature so that individual trays may be stacked and locked one upon another. These side actions add considerably to the size, cost and complexity of the typical two cavity tray mold, and the larger the size of the mold, the larger the size of the molding machine required to use it. In addition, the tonnage of the injection molding machine required to successfully mold a high impact polystyrene part is usually calculated at a minimum of two tons of machine clamping force per square inch of area in the molded part that is planar to the mold face. Therefore, a desk tray that has an inside bin dimension of 10" by 12", where the base of the bin is planar to the mold face, requires a molding machine that can generate 240 tons of clamping force to successfully mold the part. A similar two cavity mold would require a minimum 480 ton injection molding machine. Thus, to mold eight or even four desk trays in a single operation becomes economically infeasible, using these conventional molding methods, due to the large size of the molding machine required.
Previously, several constraints have prevented the production of an integrally molded multilevel vertically stacked desk paper tray. The first constraint is the need to provide adequate draft in the mold. Draft is the slight taper provided in a design of the molded part that permits the two halves of an injection mold to be removed from a cured thermoplastic molded part, without disturbing the walls of the molded part. The draft taper is provided such that as the mold is drawn away from the molded part, the mold does not encounter any portion of the part narrower than, or at a narrower angle than, any portion already encountered. It is also desirable that no very large uninterrupted surfaces be formed by a mold, to decrease the chance of disruption of the part during withdrawal of the mold.
The second constraint is the need to minimize the cycle time per molded form. The cycle time is the length of time required for positioning the mold, injecting a uncured thermoplastic material, allowing the thermoplastic material to cure, and drawing the mold away from the molded form. It is desirable if the wall thickness of the part is uniform over the part, so that it cures uniformly. The rate at which the thermoplastic material cures is determined, in part, by the thickness of the material. It is desirable, therefore, to maintain a constant thickness of material in the part while also providing adequate structural support to form a stable, solid object.
A third constraint on producing an injection moldable multilevel desk tray has been the desire to provide an backstop at the rear of each bin, as a third wall for containing tray contents within the bin. Such a design has been believed to be incompatible with the desire to integrally mold a vertically arrayed set of bins.
A fourth constraint on an injection multilevel moldable paper tray is the difficulty inherent in using a two piece, i.e. front and rear type, mold to form the top and bottom faces of each vertically stacked bin.
Because of these dual constraints, the art has been unable to design an injection moldable multilevel desk tray having a commercially acceptable cycle time. It would therefore be desirable to manufacture multilevel trays that adequately restrain the contents of each tray and that may be injection molded as a unitary form with a commercially acceptable cycle time.